Robin2:
What is missing is a complete description of the hardware you are using and the code for the programs you are using....R
Nevermind, my main problem was a misunderstanding of what those modules can do. The first practical problem I noticed is that it's relatively nonsensical to use a different UART rate compared to air rate unless you can guarantee you won't go above the air rate since the module appears to be implied by the manual-datasheet that has only 512bytes of cache (so it's a dropped-data galore that way [if you try to stream something like a big file] when it's already easy to drop data if you meet the air rate anyway).
Secondly, generally the modules are VERY low bandwidth by design so one should be careful to not try to send anything too populous [and above the air rate] (like the GNSS data I try to send; I can but I have to reduce them first). Something crucial is that they also have an error correction mode on by default (FEC) which is an EXTREME bandwidth hog so one should only enable it if they are absolutely sure they can get away with the lost bandwidth.